Process plants, like those used in chemical, petroleum, or other processes, typically include one or more centralized or decentralized process controllers communicatively coupled to at least one host or operator workstation and to one or more process control and instrumentation devices, such as field devices, via analog, digital or combined analog/digital buses. Field devices, which may be, for example valves, valve positioners, switches, transmitters, and sensors (e.g., temperature, pressure, and flow rate sensors), perform functions within the process such as opening or closing valves and measuring process parameters. The process controller receives signals indicative of process measurements or process variables made by or associated with the field devices and/or other information pertaining to the field devices via the communication buses, uses this information to implement a control routine, and then generates control signals which are sent over one or more of the buses to the field devices to control the operation of the process. Information from the field devices and the controller is typically made available to one or more applications executed by an operator workstation to enable an operator to perform desired functions with respect to the process, such as viewing the current state of the process, modifying the operation of the process, etc.
In the past, conventional field devices were used to send and receive analog (e.g., 4 to 20 milliamps) signals to and from the process controller via an analog bus or analog lines. These 4 to 20 mA signals were limited in nature in that they were indicative of measurements made by the device or of control signals generated by the controller required to control the operation of the device. However, in the past decade or so, smart field devices that perform one or more process control functions have become prevalent in the process control industry. In addition to performing a primary function within the process, each smart field device includes a memory and a microprocessor having the capability to store data pertaining to the device, communicate with the controller and/or other devices in a digital or combined digital and analog format, and perform secondary tasks such as self-calibration, identification, diagnostics, etc. A number of standard, open, digital or combined digital and analog communication protocols such as the HART®, PROFIBUS®, FOUNDATION™ Fieldbus, WORLDFIP®, Device-Net®, and CAN protocols have been developed to enable smart field devices made by different manufacturers to be interconnected within a process control network to communicate with one another, and to perform one or more process control functions.
The different function blocks within a process plant are configured to communicate with each other (e.g., over a bus) to form one or more process control loops, the individual operations of which are spread throughout the process and are, thus, decentralized. In continuous and batch control systems, the module acts as a container of measurement, calculation, and control implemented as function blocks. As defined in ISA88, a control module is a container of measurement, calculations, and control implemented as function blocks and may contain other control modules. In general, the period of execution of a module, also known as the module execution rate, may be set by the end user of the control system based on the process speed of response to changes in process inputs. As noted above, to maintain efficient operation of the overall process, and thus minimize plant shutdowns and lost profits, devices associated with the process plant must function properly and reliably. Typically, one or more experienced human operators are primarily responsible for assuring that the devices within a process plant are operating efficiently and for repairing and replacing malfunctioning devices. Such operators may use tools and applications, such as the ones described above, that provide information about devices within the process.
Analog signals from field devices are often sampled and converted to digital signals using analog to digital convertors. In sampling, a snapshot of the signal is taken at several points in time and the signal is measured at each point in time. It has been long known in communication theory that a minimum sampling rate is required to accurately process a signal. Specifically, Shannon's Theorem states that to be able to accurately represent a signal, it is necessary to sample at least two times as fast as the highest-frequency content in the signal. If a signal is sampled at a lower rate, then the signal is distorted by higher frequencies being incorrectly interpreted as lower frequencies. For example, under a stroboscopic light a rotating wheel may appear to be moving backwards. In analog input processes, as Shannon's Theorem predicts, aliasing will occur if the sample rate is not at least two times as fast as the highest frequency contained in the measurement signal. The distortion introduced by aliasing may be caused by process or electrical noise contained in the measurement signal.